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ILLINOIS  LIBRARY 

MC  URBANA  CHAMPAIGN 

BOOKSTACKS 


MODELS  OF  BLAST  FURNACES 
FOR  SMELTING  IRON 


BY 

HENRY  W.   NICHOLS 
Associate  Curator  of  Geology 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

CHICAGO 

1922 


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Field  Museum  of  Natural  History 

DEPARTMENT  OF  GEOLOGY 
Chicago,  1922 


Leaflet  Number  2 

Models  of  Blast  Furnaces  for 
Smelting  Iron 

Three  models  represent  the  hot  blast  furnace  of 
today,  the  cold  blast  furnace  of  seventy  years  ago  and 
the  small  forge  in  which  iron  was  smelted  one  hundred 
and  fifty  years  ago. 

I.  THE  MODERN  IRON  BLAST  FURNACE. 
Practically  all  the  iron  in  use  is  extracted  from  its  ore 
in  blast  furnaces,  one  of  which  is  represented  in  mini- 
ature by  the  model  in  the  right  half  of  the  case.  It  is 
called  a  blast  furnace  because  its  operation  is  main- 
tained by  a  blast  of  air  blown  in  from  below.  Its  prod- 
uct is  pig  or  cast  iron  which,  remelted  and  shaped  in 
molds,  forms  our  cast  iron  objects.  All  steel  and 
wrought  iron  is  refined  from  this  pig  iron.  The  process 
of  smelting  iron  ore  is  based  on  simple  principles  al- 
though the  practice  of  smelting  is  decidedly  complex  in 
detail.  A  mixture  of  ore,  limestone  and  fuel  is  fed  in  at 
the  top  of  a  tall  furnace  (the  furnace  reproduced  is 
sixty-five  feet  high) .  The  fuel,  coke  prepared  from  soft 
coal,  is  made  to  burn  with  great  intensity  by  a  blast 
of  hot  air  blown  in  from  below.  Hot  gases  formed  by 
the  incomplete  combustion  of  the  coke  attack  the  ore 
which  is  a  combination  of  iron  and  oxygen  and  take 
away  its  oxygen.  This  leaves  the  iron  behind  in  a 
free  state.  Lower  in  the  furnace  this  iron  absorbs 
carbon  from  contact  with  hot  coke,  melts  and  falls  to 
the  hearth  below  whence  it  is  drawn  off  from  time  to 
time  in  molten  condition.  The  earthy  impurities  of 
the  ore  and  the  ash  of  the  coal  combine  with  the  lime- 

[13] 


2  Field  Museum  of  Natural  History 

stone  and  form  a  slag  which  melts  and  runs  from  the 
furnace.  The  furnace  runs  continuously  day  and 
night  from  the  time  it  is  blown  in  until  it  is  necessary 
to  shut  down  for  repairs.  As  the  charge  wastes  away 
below  it  is  replenished  by  additions  of  ore,  limestone 
and  fuel  from  above. 

The  model  represents  in  some  detail  the  furnace 
in  which  the  smelting  is  accomplished  and  some  of  its 
accessories.  At  the  center  of  the  model  is  a  circular 
tower  which  houses  an  elevator  by  which  materials  to 
be  fed  to  the  funiace  are  lifted  to  the  top.  At  the  left 
of  the  elevator  is  a  blast  furnace  and  some  of  its  ac- 
cessories. At  the  right  the  furnace  and  accessories 
are  repeated  in  section,  that  is  cut  open  to  show  the 
inside. 

THE  FURNACE  or  STACK  is  a  tall  structure  of 
brick  enclosing  a  central  shaft  lined  with  the  best 
quality  of  fire-brick  and  cased  on  the  outside  with 
boiler  iron.  This  furnace  is  sixty-five  feet  high  and  six- 
teen feet  wide  at  its  widest  part.  Many  furnaces  are 
larger  than  this.  The  disk-like  structure  which  rests 
on  top  of  the  furnace  is  a  circular  room  of  boiler  iron 
which  encloses  the  CHARGING  PLATFORM  from 
which  the  furnace  is  fed.  The  material  fed  to  the 
furnace  is  the  CHARGE.  The  charge  is  a  mixture  in 
carefully  proportioned  quantities  of  ore,  flux  and  fuel. 
THE  ORE  is  a  combination  of  iron  and  oxygen  and  is 
the  source  of  the  iron  which  is  the  product  of  the  fur- 
nace. THE  FLUX  is  limestone.  This  combines  with 
any  infusible  matter  which  may  form  in  the  furnace, 
causes  it  to  melt  and  washes  it  from  the  furnace  in  the 
form  of  slag.  It  also  prevents  waste  of  the  iron  of 
the  ore  and  serves  other  useful  purposes.  THE  FUEL 
is  generally  coke  although  some  few  furnaces  use  char- 
coal. The  fuel  provides  the  necessary  heat  and  also 
the  reducing  gases  which  free  the  iron  from  the  ore. 

[14] 


Models  of  Blast  Furnaces  3 

The  charging  platform  is  connected  with  the  top 
of  the  elevator  tower  by  a  bridge.  Over  this  bridge  the 
charge  of  ore,  coke  and  limestone  is  wheeled  in  bar- 
rows or  small  cars.  The  top  or  THROAT  of  the  fur- 
nace opens  at  the  center  of  the  charging  floor.  This 
opening  is  ordinarily  closed  by  the  BELL,  a  conical 
valve  opening  do^vnwards.  The  barrows  or  cars  which 
bring  the  charge  are  dumped  on  top  of  the  bell.  The 
bell  is  then  momentarily  depressed  and  the  charge 
falls  into  the  furnace.  When  the  bell  is  opened  the 
furnace  fumes  come  through  to  the  charging  floor  so 
a  short  smokestack  is  provided  by  which  they  may 
escape.  The  bell  is  closed  nearly  all  the  time  and  the 
products  of  combustion  cannot  escape  through  the  top. 
They  then  pass  through  an  opening  in  the  furnace  wall 
immediately  below  the  bell  to  the  DOWN-COMER,  a 
large  brick-lined  pipe  which  passes  downward  out- 
side the  furnace.  Through  the  down-comer  the  gases 
pass  to  underground  conduits  which  conduct  them  to 
the  places  where  they  are  used.  THE  GAS  BLOW- 
OFF  is  a  smokestack  connected  with  the  down-comer 
and  closed  at  the  top  by  a  valve.  By  opening  this  valve 
any  surplus  gas  may  be  allowed  to  escape.  The  gases 
from  the  furnace  carry  with  them  dust  from  the  ore 
and  fuel  in  the  furnace.  Much  of  this  dust  settles  in 
the  DUST-CATCHER,  a  chamber  connected  with  the 
down-comer.  From  time  to  time  the  dust  is  drawn  off 
through  a  gate  in  the  bottom  of  the  dust-catcher,  and 
after  suitable  treatment,  fed  back  to  the  furnace.  The 
shaft  of  the  furnace  gradually  widens  downward  for 
more  than  half  its  length  to  the  place  where  it  is  widest 
which  is  called  the  BOSH.  From  the  bosh  it  nar- 
rows gradually  to  the  hearth  at  the  bottom.  This 
tapering  part  from  the  bosh  down  to  the  hearth  is 
called  the  BOSHES.  In  this  area  the  heat  is  so  in- 
tense that  the  fire-brick  walls  would  either  melt  or  be 

[15] 


4  Field  Museum  OF  Natural  History 

eaten  away  by  slag  if  they  were  not  protected.  Pro- 
tection is  afforded  by  water  cooling.  This  is  provided 
by  water  which  constantly  runs  through  a  number  of 
metal  pipes  imbedded  in  the  masonry  of  the  boshes. 

About  five  feet  above  the  base  of  the  furnace  and 
immediately  below  the  boshes  are  six  TUYERES. 
These  are  metal  nozzles  through  which  a  blast  of  hot 
air  continuously  enters  the  furnace.  The  tuyeres 
draw  their  supply  of  hot  air  from  the  BUSTLE  PIPE, 
a  large  brick-lined  pipe  which  encircles  the  furnace 
slightly  above  the  tuyeres.  Below  the  tuyeres  is  the 
CRUCIBLE  or  HEARTH  which  is  that  part  of  the 
furnace  in  which  the  molten  iron  accumulates  until 
there  is  sufficient  to  tap.  The  furnace  is  kept  filled 
with  a  carefully  proportioned  mixture  of  ore,  coke  and 
limestone  to  a  point  near  the  top  called  the  STOCK 
LINE.  The  blast  of  hot  air  which  enters  through 
the  tuyeres  burns  the  coke  and  in  that  part  of  the 
furnace  an  intense  heat  is  generated.  The  tall  column 
of  charge  which  lies  between  this  zone  of  combustion 
and  the  stock  line  above  rapidly  absorbs  heat  from  the 
rising  gases,  so  that  the  charge  which  is  at  an  intense 
white  heat  near  the  tuyeres  is  below  a  red  heat  at  the 
top. 

The  section  of  the  furnace  shows  the  charge  white 
hot  below,  passing  through  the  several  stages  of  yellow, 
orange  and  red  heats  to  black  at  the  top.  When  coke 
or  coal  is  burned  in  an  ordinary  stove  combustion  is 
fairly  complete  and  the  principal  product  is  an  in- 
combustible gas  called  carbon  di-oxide.  When  a  very 
deep  bed  of  fuel  is  burned,  the  combustion  is  not  com- 
plete and  the  product  is  a  gas,  carbon  mon-oxide,  which 
can  itself  be  burned.  As  the  charge  of  the  furnace 
forms  a  bed  over  fifty  feet  deep  the  gas  from  burning 
the  coke  of  the  charge  in  the  hot  blast  of  air  at  the 
tuyeres   is  this   combustible   gas,   carbon   mon-oxide. 

[16] 


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Models  of  Blast  Furnaces  5 

This  coming-  in  contact  with  hot  iron  ore  abstracts  its 
oxygen  and  frees  the  iron.  This  iron  is  set  free  in  a 
comparatively  pure  form  which  melts  with  difficulty. 
Even  in  the  intense  heat  of  the  hotter  part  of  the  fur- 
nace it  would  only  become  pasty.  In  contact  with  the 
hot  coke  each  particle  of  iron  absorbs  a  small  quantity 
of  carbon  which  gives  it  the  composition  of  cast  iron 
which  melts  readily  at  furnace  temperatures.  The 
iron  then  melts  and  drops  to  the  bottom  of  the  fur- 
nace where  it  collects  in  a  molten  slate  in  the  cruci- 
ble. When  a  sufficient  quantity  of  iron  has  collected 
it  is  withdrawn  through  the  TAP  HOLE.  This  is  a 
small  round  hole  at  the  base  of  the  furnace  connect- 
ing with  a  shallow  groove  or  channel  in  the  floor  in 
front.  This  tap  hole  is  closed  by  a  stopping  of  fire 
clay  or  other  refractory  material.  When  the  time 
comes  to  tap  the  iron,  the  clay  stopping  is  broken 
out  and  the  iron  allowed  to  run  into  the  channel  which 
may  conduct  it  to  a  pig  bed,  not  shown  in  the  model, 
where  it  is  cast  in  sand  molds  and  takes  the  form  of 
rough  blocks  called  PIGS.  When  a  steel  works  is 
connected  with  the  furnace  plant,  the  molten  iron  as 
tapped  is  conveyed  directly  to  the  mills  for  conver- 
sion to  steel. 

The  blast  furnace  runs  continuously  and  all  that 
enters  must  escape  in  a  gaseous  or  liquid  state  as  there 
is  no  provision  for  removing  solids.  Earthy  matters 
not  easily  fusible  are  present  in  the  ores  and  in  the 
ash  of  the  fuel.  Limestone  from  the  furnace  charge 
combines  with  these  and  under  the  influence  of  heat 
forms  a  fusible  waste  product,  the  SLAG  or  CINDER. 
This  floats  on  the  molten  iron  in  the  crucible  and  runs 
off  through  the  SLAG  EYE.  This  is  a  small  opening 
a  little  below  the  tuyeres  and  to  the  right  which  a  man 
with  an  iron  rod  is  cleaning.  The  blast  of  air  is  pro- 
vided by  BLOWING  ENGINES  which  do  not  appear 

[in 


6  Field  Museum  of  Natural  History 

in  the  model.  These  are  large  air-compressors  driven 
either  by  gas  engines  or  by  steam  engines  which  take 
their  steam  from  gas-fired  boilers.  These  engines  or 
boilers  use  the  furnace  gases  which  are  drawn  from  the 
top  of  the  furnace  through  the  down-comer  and  led 
to  the  engine  room  by  underground  conduits.  Other 
conduits  lead  the  compressed  air  for  the  blast  from 
the  blowing  engines  to  the  STOVES  where  it  is  heated. 
The  stoves  are  the  tall  structures  which  appear  at  the 
outer  edges  of  the  model.  They  are  built  of  fire-brick 
and  cased  with  boiler  iron.  They  are  so  designed  that 
the  inside  consists  of  numbers  of  narrow  fire-brick 
passages.  Each  furnace  is  provided  with  two  or  more 
stoves  although  only  one  appears  in  the  model.  A  pipe 
connects  the  stove  with  gas  ducts  which  are  fed  from 
the  down-comer  of  the  furnace.  Gas  is  burned  in  the 
stove  until  all  the  fire-brick  passageways  become  hot. 
Then  the  gas  is  shut  off  and  air  from  the  blowing  en- 
gine enters  at  the  top.  This  air  becomes  heated  in  the 
passage  and  emerges  below  into  underground  ducts 
which  conduct  it  to  the  bustle  pipe  of  the  furnace. 
Stoves  are  intermittent  in  operation,  one  supplies  air 
to  the  furnace  while  another  is  being  heated.  The  out- 
put of  the  furnace  is  pig  iron,  slag  and  gas.  The  gas 
is  consumed  in  the  stoves  and  in  operating  the  blowing 
engines  although  there  is  sometimes  a  surplus  which 
may  be  used  for  other  purposes.  The  slag  is  mostly 
a  waste  product  although  some  is  used  in  the  manu- 
facture of  Portland  cement  and  in  other  ways.  The 
principal  product  is  pig  iron.  This  iron  contains  small 
quantities  of  various  impurities,  which  have  an  im- 
portant effect  in  its  properties.  From  long  contact 
with  hot  coke  in  the  furnace  it  has  absorbed  a  quantity 
of  carbon  which  has  made  it  fusible  and  suited  for 
the  manufacture  of  cast  iron  articles.  Most  pig  iron 
is  refined  to  steel.     Refining  removes  impurities  and 

[18] 


Models  of  Blast  Furnaces  7 

makes  the  iron  stronger,  less  brittle,  and  more  diffi- 
cult to  melt.  When  practically  all  the  carbon  is  re- 
moved the  product  is  wrought  iron  or  mild  steel 
according  to  the  method  of  refining  employed.  When 
the  refined  product  contains  certain  definite  amounts 
of  carbon  it  is  tool  steel  or  other  high  grade  steel. 
The  output  of  a  blast  furnace  is  large.  Some  furnaces 
have  yielded  over  900  tons  of  iron  per  day.  The  fur- 
nace represented  by  the  model  yields  300  tons  daily. 

Earlier  Types  of  Furnace 

Two  models  in  the  left  half  of  the  case  represent 
two  obsolete  types  of  iron  smelting  furnaces.  One 
was  in  use  one  hundred  and  fifty  years  ago,  the  other 
sixty  years  ago  and  even  later.  They  illustrate  the 
immense  advance  in  the  art  of  iron  smelting  during 
the  past  two  hundred  years  and  call  attention  to  the 
relatively  recent  introduction  of  the  modern  furnace 
with  its  large  capacity  and  moderate  cost  of  operation. 
Before  the  time  of  the  Catalian  forge,  the  earliest  of 
those  shown,  more  primitive  devices  were  employed 
and  the  progress  from  this  forge  to  the  modern  furnace 
was  marked  by  a  number  of  furnace  types  other  than 
those  represented  by  these  models. 

II.     THE  COLD  BLAST  IRON  SMELTING 

FURNACE 

The  central  model  of  the  three  represents  a  cold 
blast  iron  smelting  furnace  modelled  on  the  same  scale 
as  the  hot  blast  furnace  to  the  right.  It  is  called  a  cold 
blast  furnace  because  the  blast  of  air  which  keeps  it  in 
operation  is  not  heated.  This  furnace  was  the  im- 
mediate predecessor  of  the  hot  blast  furnace  of  today 
which  has  completely  superseded  it.  It  was  in  general 
use  in  this  country  at  the  time  of  the  civil  war  and  for 

[19] 


8  Field  Museum  of  Natural  History 

some  years  after.  It  operated  on  the  same  principle 
as  the  modern  furnace,  but  as  it  did  not  have  the  hot 
blast,  did  not  utilize  the  furnace  gases  and  lacked  other 
refinements,  its  efficiency  and  capacity  were  low.  Its 
most  noticeable  features  were  the  massive  construction, 
the  large  flame  at  the  top  and  the  primitive  character 
of  the  equipment. 

The  massive  pyramidal  structure  near  the  center 
of  the  model  is  the  furnace.  It  is  built  of  limestone 
and  encloses  a  small  central  shaft  similar  in  outline  to 
that  of  a  modern  furnace.  It  is  built  at  the  foot  of  a 
cliff  so  that  the  charge  may  be  fed  at  the  top  without 
hoisting.  The  top  of  the  furnace  is  connected  with  the 
top  of  the  cliff  by  a  bridge  over  which  the  charge  of 
ore,  charcoal  and  limestone  is  wheeled  in  barrows.  The 
top  is  enclosed  in  an  iron  hood  which  is  provided  with 
a  smokestack  so  that  the  charging  platform  may  be 
sufficiently  free  from  gases  to  permit  men  to  work 
there.  The  top  is  open  and  the  gases  generated  in  the 
furnace  escape  and  burning  continuously  form  a  large 
flame  visible  for  miles.  Iron  and  slag  form  as  in  the 
modern  furnace  and  are  drawn  off  in  a  similar  manner 
although  the  provisions  for  this  are  more  complex  than 
they  are  in  modern  furnaces.  At  the  right  of  the  model 
is  the  wooden  blowing  engine  called  the  TUBS.  At 
the  extreme  right  is  the  water  w^heel  which  drives  the 
blovving  engine.  This  engine  consists  of  two  wooden 
upright  cylinders,  the  tubs,  provided  with  wooden 
pistons  and  appropriate  valves.  The  tubs  deliver  their 
air  to  a  third  tub  above  which  acts  as  a  receiver.  From 
this  receiver  an  underground  conduit  takes  the  air 
direct  to  the  tuyeres  which  feed  it  to  the  furnace.  The 
output  is  low.  Such  a  furnace  as  the  model  represents 
might  yield  thirty  tons  of  pig  iron  a  day. 

III.  THE  CATALIAN  FORGE.  The  model  at 
the  left  represents  the  Catalian  forge  which  was  in 

[20] 


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Models  of  Blast  Furnaces  9 

general  use  in  Europe  during  the  eighteenth  century 
and  in  this  country  at  the  time  of  the  Revolution  and 
for  some  years  after.  Forges  of  the  same  general 
type  have  been  used  among  peoples  of  the  more  primi- 
tive cultures  in  relatively  recent  times  and  they  may 
have  persisted  in  places  to  the  present  day.  The  forge 
is  small,  not  much  larger  than  an  ordinary  blacksmith's 
forge.  The  model  is  on  a  scale  of  one  inch  to  the  foot. 
At  the  left  is  the  large  hammer  which  finishes  the  iron. 
In  the  center  is  the  hearth  in  which  the  ore  is  reduced 
and  at  the  right  is  the  trompe  in  which  air  is  com- 
pressed to  provide  the  air  blast  which  urges  the  fire 
on  the  hearth. 

THE  TROMPE  which  provides  the  air  blast  for 
the  hearth  is  a  simple  device  by  which  falling  water 
is  made  to  trap  air  and  deliver  it  under  pressure.  It 
consists  of  an  upright  hollow  log  fed  with  water  from 
a  flume  above  and  connected  below  with  an  air  tight 
tank.  A  valve  at  the  top  controlled  through  a  lever 
and  hanging  cord  admits  and  shuts  off  the  water. 
Near  the  top  of  the  log  are  a  number  of  perforations. 
Water  falling  freely  draws  air  in  through  these  per- 
forations and  delivers  it  to  the  tank  below  under  a 
pressure  dependent  on  the  height  of  the  log.  The 
water  escapes  through  a  gate  near  the  bottom  of  the 
tank  and  the  air  passes  through  a  pipe  at  the  top  to 
a  conduit  which  conducts  it  to  the  hearth. 

THE  HEARTH  which  appears  in  the  center  of 
the  model  is  much  like  a  blacksmith's  hearth  with  a 
blast  from  the  tromp  instead  of  from  bellows.  On 
this  hearth  a  charcoal  fire  is  built  and  urged  to  in- 
tensity by  the  air  blast  which  enters  through  a  nozzle 
called  the  TUYERE  which  passes  through  the  rear 
wall.  This  fire  is  fed  by  a  mixture  of  iron  ore  and 
charcoal.  Under  the  influence  of  heat  the  ore,  which 
is  a  combination  of  iron  and  oxygen,  gives  up  its  oxygen 

[21] 


10  Field  Museum  of  Natural  History 

to  the  charcoal  and  metallic  iron  appears  in  the  form 
of  small  granules.  The  heat  of  the  forge  is  not  suffi- 
cient to  melt  the  iron  but  does  cause  it  to  become  pasty. 
These  pasty  grains  of  iron  settle  to  the  bottom  where 
they  cohere  and  form  a  spongy  mass  of  iron  called  the 
LOUPE.  The  pores  of  this  iron  sponge  are  filled  with 
CINDER,  a  liquid  or  pasty  mass  in  which  the  im- 
purities of  the  ore  and  the  ash  of  the  charcoal  are 
combined  with  unreduced  oxides  of  iron  from  the  ore. 
After  the  hearth  has  been  in  operation  for  some  six 
hours  the-  loupe  has  grown  to  the  proper  size  for  the 
next  treatment.  The  smelting  is  then  interrupted  and 
the  loupe  removed.  A  new  fire  is  built  and  smelting 
is  resumed. 

THE  HAMMER.  This  large  hammer  shown  at 
the  left  of  the  model  is  driven  by  a  water  wheel  con- 
cealed by  the  rear  wall  of  the  building.  The  shaft 
of  the  water  wheel  passes  through  the  wall  and  is 
armed  with  teeth  or  cams  which  act  on  the  haft  of  the 
hammer  lifting  it  and  letting  it  fall.  The  hot  loupe  is 
placed  under  this  hammer,  which  squeezes  out  the 
cinder  and  compacts  the  iron  into  a  solid  BLOOM 
which  is  merchantable.  This  forge  can  produce  either 
wrought  iron  or  steel  as  the  details  of  the  operation 
are  varied.  Cast  iron  never  forms  except  by  accident. 
The  output  of  the  forge  is  of  excellent  quality  but  the 
quantity  is  very  small  say  three  hundred  pounds  of 
iron  each  six  hours. 

Henry  W.  Nichols. 


These    models    are    exhibited    on    the    second    floor    of   the 
Museum  in  an  alcove  in  Frederick  J.  V.  Skiflf  Hall. 


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